1. Field of the Invention
The present invention relates to a near-field optical storage medium and an optical data storage system having a focusing optical system, and more particularly, to an optical storage medium which is used together with an optical pickup having a near-field focusing optical system such as a solid immersion optical system or a solid immersion lens, and a near-field optical data storage system for performing writing and/or reading of information with respect to the optical storage medium.
2. Description of the Related Art
In an optical data storage system, an optical pickup having a solid immersion optical system or solid immersion lens performs writing and/or reading of information with respect to the optical data storage medium, using a near-field formed between the solid immersion optical system or solid immersion lens and the optical data storage medium.
FIGS. 1 and 2 show an existing optical disc used as an optical data storage medium, in which FIG. 1 shows that an existing optical disc is used together with the optical data storage system having a catadioptric solid immersion optical system, and FIG. 2 shows that an existing optical disc is used together with an optical data storage system having a refractive type solid immersion lens.
In FIG. 1, a light beam 1 emitted from a light transmission and reception portion 10 is reflected by a reflective mirror 12 and incident to a catadioptric solid immersion optical system 14. A slider 16 supporting the solid immersion optical system 14 aerodynamically raises the solid immersion optical system 14 aerodynamically through an air bearing generated by a relative movement between an optical storage medium 18 such as an optical disc and the slider 16. As a result, an air gap is formed between the solid immersion optical system 14 and a protective layer 183 of the optical storage medium 18. An interval of the air gap, that is, a distance between the opposing surfaces of the solid immersion optical system 14 and the optical storage medium 18, is maintained for example within one wavelength of light used. It is preferable that it is maintained much smaller than one wavelength of the used light. The catadioptric solid immersion optical system 14 refracts and reflects the light beam 1 incident from the reflective mirror 12, and forms a beam spot focused on its surface opposing the optical storage medium 18. The beam spot forms a near field in the air gap between the solid immersion optical system 14 and the surface of the optical storage medium 18.
The optical data storage system shown in FIG. 2 includes a focusing objective lens 24 and a refractive solid immersion lens 26, instead of the catadioptric solid immersion optical system 14 shown in FIG. 1. A light transmission and reception portion 20 emits a light beam 1 having an optimized diameter for the objective lens 24. A reflective mirror 22 reflects the light beam 1 emitted from the light transmission and reception portion 20 toward the objective lens 24. The objective lens 24 focuses the light beam 1 incident from the reflective mirror 22 on the solid immersion lens 26. The beam spot focused on the solid immersion lens 26 forms a near field between a surface of the solid immersion lens 26 opposing the optical storage medium 18 and a protective layer 183 in the optical storage medium 18. The objective lens 24 and the solid immersion lens 26 are supported by a slider 28. Like the slider 16 shown in FIG. 1, the slider 28 aerodynamically raises the solid immersion lens 26 and forms an air gap having an interval within one wavelength of light used between the solid immersion lens 26 and the optical storage medium 18.
In the optical data storage system shown in FIG. 1 or 2, a beam spot is formed in a near field generating portion being a predetermined position on the surface of the solid immersion optical system 14 or the solid immersion lens 26 which opposes the optical storage medium 18. In general, the system shown in FIG. 1 or 2 uses a fine beam spot corresponding to a numerical aperture (NA) of at least one for writing or reading information with respect to the optical storage medium 18. In the case that the used light has a wavelength λ of 650 nm, a light beam which forms a beam spot on the near field generating portion passes an air gap of an interval of approximately 110 nm and a protective layer 183 of 70–90 nm thick, and is transferred to a recording layer of the optical storage medium 18. The recording layer is disposed between the protective layer 183 and a substrate 181 of the optical storage medium 18. The light beam reflected from the recording layer transmits through the protective layer 183 and the air gap and is transferred to the solid immersion optical system 14 or the solid immersion lens 26.
Generally, according to the refraction and total reflection laws, the light contributed to a large numerical aperture is totally reflected from the emergence surface of the solid immersion optical system 14 or the solid immersion lens 26, that is, the near field generating portion being an optical transmitting surface adjacent to the optical storage medium 18. Therefore, in the case that the interval of the air gap is larger than the wavelength λ of the used light, the optical storage medium 18 is positioned in the portion beyond the near field. Thus, the light contributed to the large numerical aperture does not contribute to formation of the beam spot on the optical storage medium 18. In other words, the numerical aperture of the light beam contributed to the formation of the beam spot on the optical storage medium 18 becomes smaller than “1”, while passing through the air gap. As a result, a spot size of the light beam focused on the optical storage medium 18 with the light traveling through the air gap having an interval larger than the wavelength of the used light, becomes larger than a size of the beam spot formed on the near field generating portion of the solid immersion optical system 14 or the solid immersion lens 26. However, in the case that an interval of the air gap is sufficiently smaller than one wavelength of the used light, preferably λ/4, the spot size of the light beam incident to the optical storage medium 18 is close to the size of the beam spot formed in the near field generating portion. Therefore, under this condition, the optical data storage system shown in FIG. 1 or 2 can write or read information at high density with respect to the recording layer of the optical storage medium 18, using the solid immersion optical system 14 or the solid immersion lens 26.
FIG. 3 shows the near field generating portion between the surface of the solid immersion optical system 14 or the solid immersion lens 26 and the protective layer 183 of the optical storage medium 18. The interval SRD from the surface of the solid immersion optical system 14 or the solid immersion lens 26 opposing the optical storage medium 18 to the protective layer 183, more accurately, to the recording layer, becomes smaller than one wavelength of the used light, and the recording layer in the optical storage medium 18 is positioned within the distance providing a near field effect.
An example of an existing optical disc is disclosed in U.S. Pat. No. 5,470,627. In the case that the above existing optical disc is for example a magnetooptical disc, the disc includes a reflective layer, a first dielectric layer, a recording layer, and a second dielectric layer which are disposed on a conventional substrate in sequence. The reflective layer is made of metal such as an aluminum alloy having a 500–1000 Å thickness. The first dielectric layer is made of aluminum nitride or silicon nitride having a 150–400 Å thickness. The recording layer is made of rare-earth transition-metal alloy such as TbFeCo having a 150–500 Å thickness. Finally, the protective layer is made of silicon nitride Si3N4 having a 400–800 Å thickness.
However, in the case that the above-described existing optical disc is used, the optical data storage system has two problems as follows. These problems take place identically in both the data storage system including the solid immersion optical system 14 and the data storage system including the solid immersion lens 26. Therefore, for convenience of explanation, these problems will be described in connection with the existing optical disc and the solid immersion lens 26.
First, the problem that the light beam reflected from the recording layer of the existing optical disc having the above structure contains noise due to interference will be described with reference to FIGS. 4 and 5. FIG. 4 shows the solid immersion lens 26 having a refractive index of 1.8. In FIG. 4, “air gap reflective light (NB)” illustrates the light beam totally reflected from the near field generating portion of the solid immersion lens 26 and the air gap between the solid immersion lens 26 and the optical storage medium 18, and “recording layer reflective light (RB)” illustrates the light beam reflected from the recording layer in the optical storage medium 18. In the case that the solid immersion lens 26 has a refractive index of 1.8, the total reflective angle of 56.3 degree at the solid immersion lens 26 corresponds to the numerical aperture of 0.83. FIG. 5 shows angle-reflectance characteristics of the solid immersion optical system 14 or the solid immersion lens 26 with respect to three air gap intervals. In FIG. 5, curves (a) show angle-reflectance characteristics with respect to the air gap interval of 50 nm, curves (b) show angle-reflectance characteristics with respect to the air gap interval of 100 nm, and curves (c) show angle-reflectance characteristics with respect to the air gap interval of 150 nm. Among the curves (a) through (c), the curves denoted as “++” show angle-reflectance characteristics with respect to the p-polarized light beam, and the curves denoted as solid lines show angle-reflectance characteristics with respect to the s-polarized light beam. The angle denoted at the horizontal axis indicates an incident angle possessed by the light beam proceeding to the air gap from the solid immersion lens 26. For example, in the case that an interval of the air gap existing between the optical storage medium 18 and the solid immersion lens 26 becomes larger than the wavelength of the used light, the portion of the light beam having an angle larger than the total reflection angle of 56.3 degree, particularly the portion of the light beam contributed to a higher numerical aperture, for example, the numerical aperture of 1.2 or more among the light beam proceeding from the solid immersion lens 26 to the optical storage medium 18, does not transmit through the air gap, but is totally reflected in the near field generating portion or in the inside of the air gap. As can be seen from FIG. 5 showing a reflectance with respect to the numerical aperture of 1.5, the air gap reflective light NB has a relatively higher reflectance. Also, since the air gap and the recording layer are very close to each other, an interference occurs between the air gap reflective light (NB) and the recording layer reflective light (RB). Finally, the air gap reflective light (NB) functions as noise with respect to the recording layer reflective light (RB).
Now, the problem caused by the optical storage medium 18 which is made at high density will be described with reference to FIG. 6. In the case that the optical storage medium 18 is fabricated into a high density optical storage medium, grooves or pits of 100–150 nm width are formed on a substrate 181 for recording information thereon. A reflective layer and a recording layer on which information is actually recorded are in turn put on the grooves or pits, through a coating process. In addition, a protective layer 183 of 150–200 nm thickness is formed on the recording layer. In FIG. 6, an unevenness structure 185 formed by forming the grooves or pits on the substrate 181 is shown in the form of wedges or wells. Since the depth of the recording layer coated by the protective layer 183 is larger than the width of the grooves or pits, the light beam 1 incident to the optical storage medium 18 from the solid immersion optical system 14 or the solid immersion lens 26 does not reach the grooves or pits, or more accurately, the recording layer, but is reflected in the vicinity of the inner side on the surface of the protective layer 183. As a result, the optical data storage system cannot perform writing and/or reading of information with respect to the high density optical storage medium 18.